Tagatose is a naturally occurring ketohexose and has a natural sweet taste (sweetness: 92% as sweet as sucrose) which is hardly distinguishable from sucrose and physical properties similar to sucrose. In addition, tagatose has various healthy and functional characteristics (low-calorie, antidiabetic and prebiotic effects, prevention of dental caries), and thus has been used in various products, for instance, drinks (low-calorie, supplement to bitter taste of high potency sweeteners, or reduction of blood glucose level after meal), particularly coffees (low-calorie, reduction of blood glucose level after meal, flavor enhancement), ice creams, chocolates, candies (low-calorie, non-inducing dental caries, flavor enhancement), dietary foods (low-calorie or reduction of blood glucose level after meal), foods for patients (controlling blood glucose level after meal and fasting blood glucose level, or low-calorie), and the like. In this regard, tagatose is an alternative sweetener capable of satisfying both taste and health.
However, tagatose is not often found in nature and is a rare sugar present only in small amounts in dairy products and some fruits. In order to use tagatose as a functional sweetener, it is essential to develop a method for mass production of tagatose from inexpensive raw materials.
Tagatose can be produced from galactose by enzymatic isomerization or chemical isomerization. Galactose as a substrate is a naturally occurring monosaccharide which exists in relatively small amounts, and serves as a basic constitutional unit for hydrocarbons of organisms. Typical examples of hydrocarbon components containing galactose include lactose, soybean oligosaccharides (stachyose, raffinose), agar, guar gum, arabinogalactan (consisting of few D-galactose molecules having a backbone of β-1,3 linkages with side chains of one or two L-arabinose molecules at the 6 residue), galactomannan (one of complex polysaccharides consisting of a mannose backbone of (1,4)-linkages with side chains of galactose having a 1,6-linkage), and the like. However, the preparation of tagatose by directly taking various galactose-containing hydrocarbon sources from nature is uneconomical.
Therefore, there is a high need for a more economical and efficient method for preparing tagatose.